1. Field of the Invention
The present invention relates to a single-mode fiber and to a method of manufacturing the same. More particularly, it relates to a single-mode optical fiber having a core whose refractive index distribution is nonconstant but is arbitrary and a cladding whose refractive index is constant, and a method of manufacturing the same.
2. Description of the Prior Art
With recent advances in techniques for manufacturing optical fibers, it has now become practical to employ optical fibers as communication transmission lines. Optical fibers for communication use are broadly classified into multi-mode fibers and single-mode fibers. Because of problems in manufacture, many previously used optical fibers have been multi-mode transmission fibers. However, single-mode fibers have a wider bandwidth than multi-mode fibers and can also be employed for various uses, so that future development and progress in the practical use of single mode fibers are expected.
It is well known, in theory, that a single-mode transmission fiber can be implemented as a concentric construction which consists of a core having a constant refractive index n.sub.1 and a cladding made of a material having a constant refractive index n.sub.2, that is lower than the refractive index n.sub.1. Its normalized frequency ##EQU6## where .lambda. denotes the wavelength of transmission light and a denotes the radius of the core, fulfills the following relationship: ##EQU7##
One effective method of manufacture of such an optical fiber is a method in which a core, or thin glass layers to become a cladding and a core material, is deposited on the inner wall of a substrate such as silica tube and the resultant tube is drawn at a high temperature directly or after being put into a solid preform rod to produce a fine optical fiber. Another is a method which is the rod-in-tube method and in which a rod to become a core material is inserted into a starting glass tube and the resultant tube is drawn at a high temperature similarly to the above, and so on.
In particular, a method in which a thin glass film is formed on the inner wall of a silica tube or the like by the use of the chemical vapor deposition (CVD process) and with a dopant introduced therein in order to attain a predetermined refractive index, a solid preform rod is formed and the preform rod that is thereafter drawn at a high temperature is an excellent method of manufacturing an optical fiber of low transmission loss.
Since, however, the manufacturing process includes the steps of heating and collapsing, a dip is formed in the central part of the core of the optical fiber by the preferential evaporation of the dopant introduced for controlling the refractive index of the glass, and/or a rounding of the refractive index attributed to diffusion of the dopant etc., occurs between the core and the cladding, so that the refractive index of the final optical fiber does not possess the ideal stepped index distribution which is constant in the core part. Accordingly, it becomes difficult to fabricate a single-mode fiber having the characteristics defined by Equation (1).
Especially, in a single-mode fiber, the radius a and the difference of the squares of the refractive indices, n.sub.1.sup.2 -n.sub.2.sup.2 are very small, and hence, control thereof is subject to difficulties. A previous attempt to render the refractive index of the core constant has involved removal of the dip through the selection of the material of the dopant and through a complicated control of the refractive index distribution. In both the cases, however, it is difficult to obtain a single-mode optical fiber having predetermined characteristics. Therefore, the manufactured fiber does not transmit light at the most effective wavelength or it suffers from multi-mode operation, so as to increase the transmission loss. These aspects result in the problem that the yield of production is inadequate.